1. Field of the Invention
The present invention relates to hand-held items, such as hand tools, and in particular, to devices and methods for reducing the shock and vibration caused by the use of such hand-held items.
2. Description of Related Art
Shock and vibration from the use of hand-held items, such as hand tools, are often transferred to a user""s hand through the item""s handle and grip. This may cause the user discomfort and increased fatigue. For instance, when a percussive tool, such as a hammer, strikes the surface of an object, part of the energy produced by the strike is used to perform desired work (e.g., drive a nail), part is converted into heat, and part is dissipated through the hammer. The energy that is dissipated through the hammer often produces undesirable results such as recoil of the hammer from the struck surface or excessive vibration of the hammer. The undesirable results produced by hammer strikes have been a persistent problem for the makers of hammers and other percussive tools.
Many users of hammers prefer the vibration-reducing feel of wood handled hammers, rather than integral steel handle/head hammers. A common perception is that the wood handle absorbs at least some of the shock of the hammer strikes rather than transferring all of the shock and vibration to the user""s hand, thus reducing the user""s fatigue at the end of the day as compared to using a hammer having a steel handle. However, wooden handled hammers will invariably break, typically at the wedged joint between the handle and steel head due to the prying action of nail pulling. To overcome this shortcoming, many manufacturers make integral steel handle/head hammers which hold up extremely well to nail pulling, but the shock-absorbing feature of the wood handle is lost. These problems are discussed in an article entitled xe2x80x9cNailing Basics,xe2x80x9d by Larry Haun in Fine Homebuilding, July 1997, at page 80.
In the past, various attempts have been made to reduce undesirable results produced by a hammer strike. Hammers that reduce rebound or recoil characteristics are sometimes referred to as xe2x80x9cdead blowxe2x80x9d hammers. One of the earliest attempts reflected in the prior art to produce a dead-blow hammer is U.S. Pat. No. 1,045,145, issued in November 1912 to E. O. Hubbard (xe2x80x9cHubbardxe2x80x9d). As explained by Hubbard, when the Hubbard hammer is struck against a surface, the striking head will be forced against a cushion, such that the cushion absorbs a portion of the shock of impact produced by the strike.
Following Hubbard, several other attempts were made to reduce the undesirable results of a hammer strike and, in particular, to reduce the recoil or rebound produced when a hammer strike occurs. Several early approaches for reducing recoil in hammers are summarized in U.S. Pat. No. 2,604,914 to Kahlen (xe2x80x9cKahlenxe2x80x9d) issued in July 1952. In particular, Kahlen indicates that, by 1952, known methods for reducing hammer recoil included placing either a slug, a charge of round shot, or a charge of powdered material in a chamber immediately behind a striking face of the hammer, such that the object(s) placed behind the striking head will absorb some of the forces produced by the hammer strike. The particular approach disclosed in Kahlen involved the placement of a charge of irregularly-shaped, hard, heavy particles in a chamber immediately behind the striking head of a hammer.
In addition to solutions involving cushions and charge loads, several solutions utilizing resilient members, such as elastic inserts and springs, were proposed to address the hammer strike problems, whereby a portion of the energy developed from the hammer strike is dissipated through the resilient member. Other designs, such as that disclosed in U.S. Pat. No. 5,408,902, use a xe2x80x9clagging mass,xe2x80x9d which is positioned to move towards the striking portion of the hammer head when it impacts, thus impacting the striking portion to reduce hammer recoil.
These early approaches suffer from one or more difficulties. For example, the use of slidable weights or slugs behind the striking head of the hammer is problematic because the weights themselves develop potential energy when the hammer strikes a surface and tend to recoil, thus causing undesirable vibration or oscillation of the hammer. Further, shot-filled hammers are limited: (i) because the requirement for a hollow chamber renders the size of such hammers out of proportion to their weight; and (ii) because, unless a special shot mixture is utilized, the shot is often not useful in preventing hammer recoil. Moreover, in prior art dead blow hammers, the prying and nail pulling capability of common claw hammers has been forfeited in the attempts to reduce vibration and recoil.
Further discussion of the prior art and its associated shortcomings is provided in U.S. Pat. No. 1,045,145; U.S. Pat. No. 2,604,914; U.S. Pat. No. 2,928,444; U.S. Pat. No. 4,831,901; U.S. Pat. No. 5,118,117; U.S. Pat. No. 5,408,902; and German Patent No. 1,273,449.
The present invention addresses problems associated with the prior art.
In one aspect of the present invention, a hand grip assembly adapted to be situated about a shaft of an item includes a relatively rigid shell defining an outer surface and an inner cavity. An inner elastomeric layer lines the inner cavity and defines an opening adapted to receive the shaft of the item. An outer elastomeric layer surrounds the outer surface of the rigid shell. In particular embodiments of the handle grip, the outer elastomeric layer is formed with first and second elastomeric materials, which may define different durometer hardnesses. Still further, the inner elastomeric layer may also be formed with the first elastomeric material.
In another aspect of the present invention, a shock absorbing hammer includes a hammer head including a striking portion, a shaft connected to the hammer head, and a hand grip. The hand grip has a shell defining an outer surface and an inner cavity. An inner elastomeric layer lines the inner cavity and surrounds the shaft. An outer elastomeric layer surrounds the outer surface of the shell. The outer elastomeric layer may be formed with two elastomeric materials, which may each define different durometer hardnesses. The inner elastomeric layer may be formed with one of the elastomeric materials forming the outer elastomeric layer. Moreover, in particular embodiments, the outer elastomeric layer is adapted to fit either a user""s right or left hand, for right or left handed use.
In yet another aspect of the present invention, a method of making a handle grip for an article is presented. The method includes inserting a core member into a handle shell to form an inner cavity between the core member and an inner surface of the handle shell, and situating a mold about the handle shell such that the mold seats against at least one shut-off member extending from the handle shell, so as to form at least one grip cavity and at least one handle cavity between the first mold and an outer surface of the handle shell. A liquefied first elastomeric material is injected into the inner cavity and through an opening between the inner cavity and the grip cavity, such that the first elastomeric material fills the inner cavity and the grip cavity. A liquefied second elastomeric material is injected into the handle cavity.
An alternative method of making a handle grip for an article is presented in accordance with still further aspects of the present invention. The method includes inserting a core member into a handle shell to form an inner cavity between the core member and an inner surface of the handle shell, and situating a first mold about the handle shell such that the mold seats against at least one shut-off member extending from the handle shell, so as to form at least one grip cavity between the first mold and an outer surface of the handle shell. A liquefied first elastomeric material is injected into the inner cavity and through an opening between the inner cavity and the grip cavity, such that the first elastomeric material fills the inner cavity and the grip cavity. The first mold is removed from the handle shell, and a second mold is situated about the handle shell such that the mold seats against the shut-off member so as to form at least one handle cavity between the second mold and the outer surface of the handle shell. A liquefied second elastomeric material is then injected into the one handle cavity.